Terephthalic acid is produced by a liquid-phase oxidation reaction of p-phenylene compounds such as p-alkylbenzenes typified by p-xylene. The production of terephthalic acid generally uses acetic acid as a solvent and a catalyst such as cobalt and manganese, optionally with an added promoter such as bromine compounds or acetaldehyde. Since acetic acid is used as the solvent as mentioned above, the crude terephthalic acid slurry obtained through the liquid-phase oxidation reaction contains a large amount of impurities such as 4-carboxybenzaldehyde (4CBA), para-toluic acid (p-TOL), benzoic acid, and various other coloring impurities. Therefore, the crude terephthalic acid obtained by being separated from the slurry also contains these impurities, and a highly sophisticated purification technique is needed to obtain high-purity terephthalic acid.
Various methods for purifying crude terephthalic acid are known, such as a method in which terephthalic acid is dissolved in acetic acid, water, or mixed solvents thereof at high temperature and high pressure and the solution is then subjected to catalytic hydrogenation, decarbonylation, oxidation, or recrystallization and a method in which slurry having terephthalic acid crystals partially dissolved therein is subjected to high-temperature immersion. Both the production of crude terephthalic acid by a liquid-phase oxidation reaction and the purification of the crude terephthalic acid require a final procedure for separation of the terephthalic acid crystals from the dispersion medium.
Oxidation intermediates such as 4CBA, p-TOL, and benzoic acid or coloring substances which are present as impurities in the oxidation-derived slurry or the slurry resulting from purification of crude terephthalic acid are dissolved in the dispersion medium of the slurry at high temperature. Thus, when the slurry is cooled to around 100° C. to give slurry containing terephthalic acid crystals, these impurities are incorporated in the terephthalic acid crystals, with the result that it is difficult to obtain high-purity terephthalic acid.
Therefore, in order to separate terephthalic acid as pure as possible from the dispersion medium of the crude terephthalic acid slurry resulting from the oxidation reaction or the slurry resulting from the purification of crude terephthalic acid, the separation should be conducted under high-temperature, pressurized conditions. The method that is most commonly used to separate a dispersion medium from slurry containing crystals is centrifugation. The centrifugation is widely used also for slurries resulting from oxidation reaction and for slurries resulting from purification. The centrifugation is a method in which the slurry is introduced into a basket rotating at a high speed so as to cause the dispersion medium to overflow from the top of the basket while directing the crystals to the bottom of the basket. However, continuous operation at high temperature and high pressure is known to pose some difficulties arising from the structural and functional limitations of the centrifuge.
First, the crystals are difficult to rinse during the centrifugation or after the separation, and thus the amount of the dispersion medium adhered to the crystals tends to increase. A common method employed to solve this problem is to form a cake of the centrifugally separated terephthalic acid crystals into slurry with a fresh, hot solvent. This method, however, has the disadvantage of requiring several repetitions of the separation procedure. Furthermore, owing to the high-speed rotation at high temperature and high pressure, the maintenance of the centrifuge is so cumbersome and difficult that the cost of maintenance increases. The centrifugation cannot therefore be considered sophisticated as a technique for use in this field.
As a separation technique alternative to the centrifugation, a dispersion medium replacement apparatus making use of gravitational sedimentation of terephthalic acid crystals has been proposed. For example, Patent Literature 1 and Patent Literature 2 propose such apparatuses. According to Patent Literature 1, a lateral shelves with a plurality of holes are provided inside the dispersion medium replacement apparatus. It is stated that without such a structure, the efficiency of the replacement would be unsatisfactory due to channeling or back mixing of the fluid in the apparatus. In Patent Literature 2, a shelf plate forming a slope is provided to improve the replacement performance. However, when slurry is treated, in particular when slurry is subjected to dispersion medium replacement making use of gravitational sedimentation, the provision of such a shelf plate entails significant difficulty. Specifically, the slurry is sedimented on the shelves and clogs the holes, which requires a tremendous effort to stabilize the operation. These techniques can therefore never be considered sophisticated.
Dispersion medium replacement apparatuses requiring no shelf plates have also been proposed (Patent Literatures 3 and 4, for example). Such a dispersion medium replacement apparatus requiring no shelf plates has the following four inlets/outlets: (1) a supply port through which starting slurry consisting of a first dispersion medium and terephthalic acid crystals is supplied to an upper compartment of the apparatus; (2) a supply port through which a second dispersion medium is introduced into a lower compartment of the apparatus; (3) an outlet port through which mainly replaced slurry consisting of terephthalic acid crystals and the second dispersion medium is discharged from the lower compartment of the apparatus; and (4) an outlet port through which mainly the first dispersion medium is discharged from the upper compartment of the apparatus. The flow rates through these ports can be freely changed, except for the supply flow rate through the port (1). This provides flexibility of operation; however, control of the flow rates is considerably complicated since changing the flow rates influences the performance properties such as the efficiency of dispersion medium replacement. Thus, for example, Patent Literature 3 discloses that the dispersion medium replacement apparatus can easily be allowed to continue stable operation at a high replacement efficiency by adjusting the temperature distribution inside the dispersion medium replacement apparatus so as to render its upper part hotter and create a zone showing a sharp change in temperature and by controlling the amount of the second dispersion medium to be introduced and/or the amount of the replaced slurry to be discharged so as to keep this temperature zone at a desired position.
Patent Literature 4 mentioned above discloses that the change rate of terephthalic acid content per unit thickness (mass %/m) in a boundary region needs to be controlled to be 15 or more but 500 or less in order to significantly improve the replacement efficiency. However, when the amount of the second dispersion medium to be introduced and/or the amount of the replaced slurry to be discharged is controlled on the basis of the change rate of terephthalic acid content, it is necessary to know the terephthalic acid content in the dispersion medium replacement apparatus.
The terephthalic acid slurry supplied to the dispersion medium replacement tower is in the form of an aqueous slurry at around 140 to 190° C., while water at around 100° C. is supplied as the second dispersion medium from the bottom of the tower. Thus, the interior of the dispersion medium replacement apparatus is pressurized, and the density of terephthalic acid is difficult to measure through sampling depending on the height of the boundary region. Patent Literature 3 also discloses a method of detecting the terephthalic acid content in slurry with the aid of an on-line densitometer disposed in the pipe of the above outlet port (3) through which mainly the replaced slurry consisting of terephthalic acid crystals and the second dispersion medium is discharged or in the pipe designed to circulate the slurry into the dispersion medium replacement tower.